Standard chip carriers are nonmetal in construction and are made chiefly of either glass or ceramic. To adapt a standard ceramic chip carrier for microwave applications, most of the entire outside surface would have to be metallized for shielding purposes and closure would have to be effected by solder sealing of the cover or lid. This closure method can result in solder being pressed into the inside of the package, or balls of solder may be formed during the soldering process, with the solder balls being free floating inside the package.
Moreover, the impedance of these standard chip carriers can only be controlled by the thickness of ceramic or glass between the layers of metallization and typically no shielding is provided between leads. Thus standard chip carriers in their present form cannot be used to house microwave devices.
It is nonetheless desirable to be able to modify standard glass or ceramic chip carriers so that these standard chip carriers can be used at microwave frequencies. More specifically, it would also be of definite advantage to be able to weld a cover onto these packages for reasons of repairability and hermeticity. Standard chip carriers are hermetically sealed in such a manner that once sealed, the package must be destroyed in order to gain access to the components housed in the package. In contradistinction, welded packages have the advantage that the welded lid can be removed by grinding the weld off to permit access to the component, with the lid then being rewelded after repair.
Microwave devices are presently housed in modules which are either routed out to provide a cavity for the device, or the housing is made up of metal parts which are brazed or soldered together. While Kovar is presently used for microwave modules, the lids of these devices are brazed or soldered, as opposed to welded, primarily because of the use of relatively large-diameter glass feedthroughs. These glass feedthroughs are on the order of 80 mils in diameter which results in an overall minimum package height of 110 mils, since the use of the glass feedthroughs requires 15 mils of sidewall material to either side. As used herein, mils refers to thousandths of an inch. In order to make these packages weldable, the frame height would have to be on the order of 135 mils. This means that 55 mils are added for accommodating the welding process. When a 20-mil base is added with a 5-mil cover, the resulting overall package height is on the order of 160 mils. This height is excessive and would preclude the use of such a weldable all-metal package in most if not all applications.